1. Field of the Invention
The present invention relates to a multi-domain liquid crystal display, more specifically to a multi-domain liquid crystal display with high contrast ratio and quick response.
2. Description of the Related Art
A liquid crystal display device is currently used as a flat panel display that provides images comparable to a CRT (Cathode Ray tube) in a personal computer (PC), a television (TV) receiver, a display screen for cellular phone, various OA (Office Automation) equipments and the like. Further growth of the liquid crystal display is anticipated.
Generally, a liquid crystal display device comprises a pair of substrates having liquid crystal filled in the gap between the substrates and adjusts the amount of light that is transmitted through the liquid crystal in response to the electric field applied thereto. A vertically aligned (VA) twisted-nematic type of the liquid crystal display device comprises a pair of transparent substrates having a transparent electrode on the inner surfaces and polarizing plates on the outer surfaces. A liquid crystal material having negative dielectric anisotropic property is filled in the gap between the pair of transparent substrates. When a voltage is applied between the electrodes, an electric field is developed in the vertical direction to the substrates, thereby causing longer axes of the liquid crystal molecules to slant in the direction perpendicular to the electric field and twisting asymmetric molecules of the liquid crystal in a spiral manner at a constant pitch.
Since liquid crystal molecules near an alignment film are substantially perpendicular even if no voltage is applied, display quality of a VA type liquid crystal display is significantly better than that of an TN (Twisted-Nematic) type liquid crystal display in contrast and viewing angle characteristic. However, it has disadvantages of degrading contrast when the screen is viewed at an off-angle and narrower viewing angle performance due to inversed display or the like. A multi-domain technology has been developed to improve such disadvantages of the VA type liquid crystal display, in which alignment of liquid crystal molecules is divided into plural different orientations within a pixel.
In the multi-domain technology, longer axes of liquid crystal molecules that are vertically aligned in case of no voltage application tend to fall when a voltage is applied. However, falling directions of liquid crystal molecules are random and thus not regulated, thereby making it difficult to perform a uniform optical characteristic and exhibiting a slow response characteristic. Various attempts have been made to improve response time by regulating the moving direction of liquid crystal molecules at an early stage of voltage application. For example, in the Patent Document 1 as listed hereunder, protrusions (or projections) having sloped side surfaces are formed in respective pixel areas, thereby achieving a multi-domain configuration by utilizing the phenomenon that liquid crystal molecules fall under the influence of the sloped surfaces of the protrusions when a voltage is applied to the liquid crystal layer. On the other hand, it is proposed in the Patent Document 2 as listed hereunder to form an opening at the interfacing portion at the center of each pixel electrode in the opposed electrodes, thereby developing a portion of sloped electric field at the center portion of each pixel. As a result, alignment of each liquid crystal molecule is split into two or four orientations.
However, in the method of providing the protrusions, liquid crystal molecules near the protrusions are sloped even in case of applying no voltage to such pixel electrode, thereby reducing contrast due to light leakage under the intrinsically black condition of the display screen. It also reduces the response time because of gradual slope of liquid crystal molecules near the protrusion when a voltage potential is applied. Moreover, there causes variations in the way of falling liquid crystal molecules unless otherwise precisely controlling the width, height, and slope angle of each protrusion, thereby resulting in non-uniformity in the half-tone zone and making the fabrication process expensive. On the other hand, in the method of forming the opening in each pixel electrode, since no voltage is applied to the areas where no electrodes are formed, response times of the liquid crystal become slow in display transitions from black to half-ton or from white to half-tone, thereby increasing driving voltage and power consumption.    [Patent Document 1] Japanese patent publication no. 2003-167253    [Patent Document 2] Japanese patent publication no. 6-301036